Staff covering large garden at the back getting fertiliser (6)
I believe the answer is:
pollen
'fertiliser' is the definition.
(pollen fertilises plants)
'staff covering large garden at the back' is the wordplay.
'staff' becomes 'pole' (I've seen this before**).
'covering' indicates putting letters inside.
'large' becomes 'l'.
'at the back' says to take the final letters.
The final letter of 'garden' is 'n'.
'pole' placed around 'l' is 'polle'.
'polle'+'n'='POLLEN'
'getting' is the link.
(Other definitions for pollen that I've seen before include "Powdery substance given off by plants" , "Frequent cause of allergies" , "Fertilising grains" , "Fine powder given off by plants" , "Fertilising powder from plants" .)